High Al-content steel sheet excellent in workability and method of production of same

ABSTRACT

The present invention provides a high Al-content steel sheet having an excellent workability and a method of production of the same at a low cost by mass production, a high Al-content metal foil and a method of production of the same, and a metal substrate using a high Al-content metal foil, that is, a high Al-content steel sheet having an Al content of 6.5 mass % to 10 mass %, the high Al-content steel sheet characterized by having one or both of a {222} plane integration of an α-Fe crystal with respect to the surface of the steel sheet of 60% to 95% or a {200} plane integration of 0.01% to 15% and a method of production of the same, a high Al-content metal foil and a method of production of the same, and a metal substrate using a high Al-content metal foil.

TECHNICAL FIELD

The present invention relates to a high Al-content steel sheet and metalfoil and methods of production of the same.

BACKGROUND ART

An Al-containing steel sheet comprised of iron plus aluminum is an alloyhaving a high temperature oxidation resistance and a high electricalresistance. Raising the Al concentration can be expected to improvethese properties, but simultaneously the toughness falls and theworkability remarkably drops. For this reason, the maximum Alconcentration at which industrial mass production is possible in theordinary steel-making and rolling process is just 6.5 mass %. Massproduction technology for improving the workability and producing highAl-content steel sheet containing more Al is eagerly awaited.

As prior art for improving the workability of a high Al-content steelsheet, Japanese Patent Publication (B) No. 6-8486 describes addingprescribed amounts of P and REMs to improve the hot workability sincethe Ce in REMs is present in steel as a fine grain phosphate. However,this requires expensive additive elements and leads to higher costs.Further, the upper limit of the Al concentration described in thispublication is 6.5 mass %.

Japanese Patent Publication (A) No. 1-4458 describes making 70% or moreof the solidified structure of rapidly quenched ribbon columnar crystalsso as to improve the toughness of stainless steel containing Al in aconcentration of 1 to 7 mass %. However, considering the fact thatspecial equipment is necessary for producing rapidly quenched ribbon andfor the mass producibility of ribbon, the cost again ends up becominghigher.

Therefore, as a method of production of a high Al-content steel sheet,the method of depositing an Al or Al alloy on the surface of a steelsheet having a concentration of Al of less than 6.5 mass % able to bemass produced by the usual steel-making and rolling process and treatingthis by diffusion heat treatment to increase the Al concentration isknown.

For example, Japanese Patent Publication (B) No. 4-80746 discloses, asart for efficiently rolling an Al plated material, a method ofproduction of a metal substrate comprising plating the surface of asteel sheet with Ni, then plating it by Al or an Al alloy, rolling it tothe thickness of the final product, then working it and assembling itinto a metal substrate and treating it by diffusion heat treatment.Further, Japanese Patent Publication (A1) No. 2003-520906 describes, asa method of production of dimensionally stable Fe—Cr—Al foil steelsheet, coating it with Al or an Al alloy during which making the amountof coating 0.5 to 5 mass % of the entire mass so that the shrinkage inthe length and/or width direction becomes 0.5% or less even in theuniform diffusion heat treatment after cold rolling. However, thesemethods of production do not aim at the effect of improvement of thetoughness or workability of the high Al-content steel sheet afterdiffusion heat treatment.

Japanese Patent No. 3200160 describes, as a method of production of anFe—Cr—Al alloy foil, depositing Al or an Al alloy on a stainless steelsheet containing a prescribed amount of B, treating it by diffusion heattreatment to make the deposited ingredient diffuse in the steel sheetand obtain a steel sheet containing Al in a concentration of 1 to 10mass %, then cold rolling it by a reduction rate of 30% or more andannealing it in a range of 800° C. to 1200° C. one time or more so as topromote the uniform diffusion of B and improve the resistance to hightemperature embrittlement. With this method of production, the coldrolling and annealing are repeated after the diffusion heat treatment,so the cost becomes higher.

DISCLOSURE OF THE INVENTION

As explained above, the methods disclosed in Japanese Patent Publication(B) No. 6-8486 and Japanese Patent Publication (A) No. 1-4458 improvethe workability by using special additive elements or a rapid quenchingapparatus and therefore result in higher costs. The methods ofdepositing Al or an Al alloy on a steel sheet and treating it bydiffusion heat treatment to enrich the Al concentration disclosed inJapanese Patent Publication (B) No. 4-80746, Japanese Patent Publication(A1) No. 2003-520906, and Japanese Patent No. 3200160 are mainlydisclosed as methods of production of catalyst substrates for exhaustgas purification (metal substrates) and are not art for improving theworkability of high Al-content steel sheet after diffusion heattreatment. For this reason, while it is possible to deposit Al or an Alalloy on steel sheet, then treat it by diffusion heat treatment toproduce high Al-content steel sheet, there was then the problem of a lowworkability and therefore breakage when rolling, bending, or otherwiseworking high Al-content steel sheet.

The present invention solves these problems by providing a highAl-content steel sheet having excellent workability and a method ofproduction of the same at low cost by mass production, a high Al-contentmetal foil and method of production of the same, and a metal substrateusing a high Al-content metal foil.

The inventors solved this problem by discovering that it is possible toimpart excellent workability by controlling a high Al-content steelsheet having an Al content of 6.5 mass % to 10 mass % to a specifictexture structure. Further, they discovered that by depositing Al or anAl alloy on a matrix steel sheet containing Al in an amount of 3.5 mass% to less than 6.5 mass % to obtain a laminate and cold rolling thislaminate so as to impart working strain and then treating this bydiffusion heat treatment to make the Al or Al alloy deposited on thesurface diffuse in the steel sheet, it is possible to control thetexture structure of the high Al-content steel sheet after diffusionheat treatment to the range of the present invention and as a resultgreatly improve the workability. The present invention has as its gistthe following.

(1) A high Al-content steel sheet having an Al content of 6.5 mass % to10 mass %, the high Al-content steel sheet characterized by having oneor both of a {222} plane integration of 60% to 95% or a {200} planeintegration of 0.01% to 15% of the α-Fe crystal with respect to thesurface of the steel sheet.

(2) A high Al-content steel sheet as set forth in (1), characterized inthat the steel sheet has a thickness of 0.015 mm to 0.4 mm.

(3) A high Al-content metal foil having an Al content of 6.5 mass % to10 mass % and a steel sheet thickness of 0.005 mm to 0.1 mm, the highAl-content metal foil characterized by having one or both of a {222}plane integration of 60% to 95% or a {200} plane integration of 0.01% to15% of the α-Fe crystal with respect to the surface of the steel sheet.

(4) A honeycomb structure comprised of a flat foil and corrugated foilof a metal foil superposed and wound up, the honeycomb structurecharacterized in that at least part of the metal foil is a highAl-content metal foil as set forth in (3).

(5) A metal substrate obtained by brazing together a honeycomb structureas set forth in (4) and a metal outer jacket.

(6) A metal substrate obtained by brazing together a honeycomb structureas set forth in (4) and a metal outer jacket, then carrying a catalystlayer.

(7) A method of production of high Al-content steel sheet excellent inworkability characterized by comprising depositing Al or an Al alloy onat least one surface of a matrix steel sheet containing Al in an amountof 3.5 mass % to less than 6.5 mass % to obtain a laminate, impartingworking strain to the laminate by cold working, then treating it bydiffusion heat treatment.

(8) A method of production of high Al-content steel sheet as set forthin (7), wherein the method of deposition of the Al or Al alloy is hotdipping of Al or an Al alloy on the steel sheet.

(9) A method of production of high Al-content steel sheet as set forthin (7), wherein the method of deposition of the Al or Al alloy is themethod of attachment of a foil of Al or an Al alloy to a steel sheet.

(10) A method of production of high Al-content steel sheet as set forthin (7), wherein the method of imparting working strain by cold workingis cold rolling.

(11) A method of production of high Al-content steel sheet as set forthin (10), wherein a reduction rate α(%) of the laminate in the coldrolling is in a range satisfying the relations of the following (a) to(c):20≦α≦95  (a)35x−260≦α  (b)6.5≦x  (c)

-   -   (where, x is the average Al concentration of the steel sheet        after diffusion heat treatment (mass %))

(12) A method of production of high Al-content steel sheet as set forthin (10) or (11), wherein a thickness t (mm) of the laminate after coldrolling is in a range satisfying the relations of the following (d) to(f):0.015≦t≦0.4  (d)t≦−0.128x+1.298  (e)6.5≦x  (f)

-   -   (where, x is the average Al concentration of the steel sheet        after diffusion heat treatment (mass %))

(13) A method of production of high Al-content steel sheet as set forthin (7), wherein the diffusion heat treatment is performed at atemperature of 800 to 1250° C.

(14) A method of production of high Al-content steel sheet as set forthin (7) or (13), wherein the diffusion heat treatment is performed in anatmosphere of a non-oxidizing atmosphere.

(15) A method of production of high Al-content steel sheet as set forthin any one of (7) to (14), further comprising preheating the steelsheet, then forming the laminate.

(16) A method of production of high Al-content steel sheet as set forthin (15), wherein the preheating is performed at a temperature of 700 to1100° C.

(17) A method of production of high Al-content steel sheet as set forthin (15) or (16), wherein the preheating is performed in an atmosphere ofat least one of a vacuum, inert gas atmosphere, or hydrogen atmosphere.

(18) A method of production of high Al-content metal foil characterizedby further cold rolling a high Al-content steel sheet obtained by amethod of production as set forth in any of (7) to (17).

(19) A high Al-content metal foil obtained by a method of production asset forth in (18), the metal foil characterized in that the foilthickness is 5 to 100 μm.

(20) A honeycomb structure comprised of a flat foil and corrugated foilof a metal foil superposed and wound up, the honeycomb structurecharacterized in that part of the metal foil is a high Al-content metalfoil as set forth in (19).

(21) A metal substrate obtained by brazing together a honeycombstructure as set forth in (20) and a metal outer jacket.

(22) A metal substrate obtained by brazing together a honeycombstructure as set forth in (20) and a metal outer jacket, then carrying acatalyst layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the claims as defined by the reduction rate of the laminateand Al concentration (equations in figure show line segments)

FIG. 2 shows the claims as defined by the laminate thickness after coldrolling and the Al concentration (equations in figure show linesegments)

BEST MODE FOR CARRYING OUT THE INVENTION

Below, the present invention will be explained in more detail.

The inventors searched for a method for improving the workability withrespect to bending, drawing, rolling, etc. in a high Al-content steelsheet with an Al content of 6.5 mass % to 10 mass % and discovered thatby controlling the texture structure of the steel sheet to a specificrange, cracks and breakage can be suppressed extremely effectively.

If the high Al-content steel sheet has an Al content of less than 6.5mass %, even without the texture structure of the present invention,excellent workability is obtained. If over 10 mass %, the effects ofintermetallic compounds etc. appear and even with the texture structureof the present invention, excellent workability can no longer beobtained. Therefore, the Al content was made 6.5 mass % to 10 mass %.

The texture structure of the present invention satisfies either thecondition of the {222} plane integration of the α-Fe crystal withrespect to the surface of the steel sheet being an extremely high levelof 60% to 95% or the {200} plane integration being an extremely lowlevel of 0.01% to 15%.

Here, the plane integration is measured by X-ray diffraction byMoKα-rays. Explained this in more detail, each sample was measured forthe integrated intensity of 11 orientation planes of the α-Fe crystalparallel to the sample surface ({110}, {200}, {211}, {310}, {222},{321}, {411}, {420}, {332}, {521}, {442}), then each measured value wasdivided by the theoretical integrated intensity of a sample of a randomorientation, then the ratio of the {200} or {222} intensity was found asa percentage. This is shown by the following equation (I) by for examplethe {200} intensity ratio.{200} plane integration=[{i(200)/I(200)}/Σ{i(hkl)/I(hkl)}]×100  (I)

where the symbols mean the following:

i(hkl): Actual measured integrated intensity of {hkl} plane at measuredsample

I(hkl): Theoretical integrated intensity of {hkl} plane at sample havingrandom orientation

Σ: Sum for 11 orientation plans of α-Fe crystal

Here, the integrated intensity of the sample having a random orientationmay be found by preparing and measuring a sample.

If the high Al-content steel sheet has a {222} plane integration of lessthan 60% and the {200} plane integration becomes 15% or more, cracks orbreakage easily occur at the time of drawing, bending, and rolling.Further, if the {222} plane integration exceeds 95% and the {200} planeintegration is less than 0.01%, the effect becomes saturated andproduction becomes difficult. Therefore, the high Al-content steel sheetof the present invention is given the above texture structure.

The high Al-content steel sheet of the present invention preferably hasa thickness of 0.015 mm to 0.4 mm. If the thickness is less than 0.015mm, the production yield falls, so the lower limit of the steel sheetwas made 0.015 mm. If over 0.4 mm, the texture structure of the presentinvention becomes hard to obtain and the workability drops, so the upperlimit of the thickness was made 0.4 mm.

Further, surprisingly, the inventors discovered that when cold rollingthe high Al-content steel sheet of the present invention to a metal foilof a thickness of 5 to 100 μm, the texture structure of the steel sheetis maintained between rollings and that the texture structure after coldrolling maintains a state whether either the {222} plane integration ofthe α-Fe crystal with respect to the surface of the metal foil surfaceis the extremely high level of 60% to 95% or the {200} plane integrationis the extremely low level of 0.01% to 15% and the workability isexcellent. For this reason, the high Al-content metal foil of thepresent invention can be easily bent, drawn, or otherwise worked withoutheat treatment or other softening treatment due to the effects of thetexture structure regardless of being as cold rolled.

If the high Al-content metal foil after cold rolling has a {222} planeintegration of less than 60% and a {200} plane integration of 15% ormore, cracks and breakage easily occur at the time of drawing, bending,and rolling. Further, if the {222} plane integration becomes over 95%and the {200} plane integration becomes less than 0.01%, the effectbecomes saturated and, further, production becomes difficult. Therefore,the high Al-content metal foil of the present invention is given thefollowing texture structure. Here, the plane integration is measured inthe same way as the method explained above.

When using a high Al-content metal foil as the material for a metalsubstrate described below, the range of the foil thickness is preferablymade 5 to 100 μm. This is because if the foil thickness is less than 5μm, the resistance to high temperature oxidation falls, while if over100 μm, the wall thickness becomes larger and thereby the pressure lossof the exhaust gas becomes larger. This is not preferable as a catalystsubstrate for exhaust gas purification. A more preferable foil thicknessis 10 to 60 μm.

A honeycomb structure obtained by corrugating a high Al-content metalfoil, then winding up a flat foil and corrugated foil increases thecontact area with the exhaust gas and improves the exhaust gaspurification ability. The metal foil of the present invention isexcellent in workability, so can be easily corrugated and formed into ahoneycomb structure as cold rolled and has excellent dimensionalprecision of working, so has small variation in corrugated height etc.

When using a honeycomb structure as the metal substrate, for example anNi-based brazing material is used to braze a metal outer jacket over thehoneycomb structure. The brazing is performed by placing a brazingmaterial at the contact points of the corrugated foil and flat foil andbetween the honeycomb structure and outer jacket, then heating in avacuum at 1000 to 1200° C. or so. A honeycomb structure using the highAl-content metal foil of the present invention is also excellent inbrazeability and excellent in structural durability after brazing. As aresult, the high Al-content metal foil and honeycomb structure producedaccording to the present invention are excellent in workability andsimultaneously are excellent in toughness as well. When used as a metalsubstrate in exhaust gas, there is less cracking or breakage of thefoil. Further, when depositing a catalyst layer on the metal substrateof the present invention, since the metal substrate has a highstructural endurance, there is little delamination of the catalyst layerand therefore little drop in purification ability of exhaust gas at thetime of use.

Next, a method of production of a high Al-content steel sheet of thepresent invention will be explained in detail below.

The inventors discovered that by depositing Al or an Al alloy on amatrix steel sheet containing Al in an amount of 3.5 mass % to less than6.5 mass % to obtain a laminate, cold rolling this laminate, thentreating it by diffusion heat treatment, it is possible to produce highAl-content steel sheet having Al in an amount of 6.5 mass % or more andthat this high Al-content steel sheet has the texture structure of thepresent invention and has excellent workability. In particular, theydiscovered that by defining the reduction rate at the time of coldrolling the laminate and the thickness after cold rolling according tothe desired concentration of Al in the steel sheet after the diffusionheat treatment, an excellent workability can be obtained and a highAl-content steel sheet can be easily worked. Further, they discoveredthat by preheating the steel sheet before depositing the Al or Al alloy,the effect of realizing excellent workability is remarkably improved.The reason is believed to be that by depositing Al or an Al alloy on amatrix steel sheet having 3.5 mass % to less than 6.5 mass % of aconcentration of Al to obtain a laminate and cold rolling this laminate,the dislocation structure of the laminate and the interface structure ofthe matrix steel sheet and Al or Al alloy layer differ from the caseobtained by the conventional method and as a result the microstructureof the steel sheet after diffusion heat treatment to increase the Alconcentration changes and the workability is improved. Further, it isbelieved due to the preheating, the structural changes for realizingexcellent workability acts more effectively.

Below, the limitative conditions of the present invention will beexplained.

For the matrix steel sheet on which the Al or Al alloy is deposited, asteel sheet containing Al in an amount of 3.5 mass % to less than 6.5mass % is used. This is because if including 3.5 mass % or more of Al inthe matrix steel sheet in advance, the steel sheet is improved inworkability after diffusion heat treatment. Further, the reason formaking the Al concentration less than 6.5 mass % is that, as explainedin the background art, steel sheet containing 6.5 mass % or more of Alhas a low toughness and poor workability, so mass production isdifficult. For this reason, the upper limit of the matrix steel sheetwas made less than 6.5 mass %. The more preferable range of Alconcentration of the matrix steel sheet is 4.2 to 6.0 mass %. Further,for the steel sheet forming this matrix, alloy ingredients able to givethe mechanical characteristics or chemical characteristics sought in thefinal product are selected as other elements. For example, if a hightemperature oxidation resistance is necessary, a ferrite-based stainlesssteel to which a suitable quantity of Cr or a rare earth element isadded is selected. When a high strength is sought, a steel sheet towhich an element enabling realization of solution strengthening orprecipitation strengthening is added is used.

The Al or Al alloy is deposited on the steel sheet by hot dipping,electroplating, the dry process, the ground process, etc. No matter bywhich method it is deposited, the effect of the present invention can beobtained. Further, it is possible to add the desired alloy elements tothe Al or Al alloy to be deposited and alloy them simultaneously withthe increase in Al concentration. The amount of the Al or Al alloy to bedeposited is determined in accordance with the thickness of the matrixsteel sheet at the time of deposition, the concentration of Al in thesteel sheet, and the desired concentration of Al after diffusion heattreatment. For example, when depositing an Al alloy of 90 mass % Al-10mass % Si on both surfaces of a matrix steel sheet of a thickness of 0.4mm having a 5 mass % Al concentration and making the concentration of Alafter diffusion heat treatment 8.1 mass %, the thickness of depositionof the Al alloy per side is about 20 μm.

The present invention further features optimizes the conditions for coldrolling the laminate obtained by depositing Al or an Al alloy on amatrix steel sheet containing Al in an amount of 3.5 mass % to less than6.5 mass % in accordance with the average concentration of Al of thesteel sheet after the diffusion heat treatment. The laminate is coldrolled to change the dislocation structure of the laminate and theinterface structure of the matrix steel sheet and Al or Al alloy. Due tothis cold rolling, the steel sheet is remarkably improved in workabilityafter the Al diffusion heat treatment. As explained above, this effectis remarkable when the concentration of Al of the matrix steel sheet is3.5 mass % to less than 6.5 mass %. The inventors discovered that thereis an optimum reduction rate of cold rolling of the laminate andthickness after cold rolling dependent on the concentration of Al afterdiffusion heat treatment and thereby completed the present invention.

The range of the reduction rate α(%) of the laminate in whichparticularly preferable effects can be obtained in the present inventionmay be illustrated as in FIG. 1. Here, the x (mass %) is the average Alconcentration after diffusion heat treatment. The reason for making thereduction rate α(%) 20≦α≦95 is that, regardless of the Al concentration,if the reduction rate is less than 20%, since the amount of the workingstrain imparted is small, a large effect of improvement of theworkability cannot be obtained. If over 95%, not only can no furtherimprovement in the workability be recognized, but also the problemarises of partial delamination of the Al. The reason for the 40x−300≦αis that the higher the Al concentration, the lower the toughness of thesteel sheet after diffusion heat treatment and the lower theworkability, so to realize the effect of the present invention of theexcellent workability, the higher the Al concentration, the larger thereduction rate must be made. Further, the present invention covers themethod of production of steel sheet containing a high concentration ofAl of 6.5 mass % or more, which is difficult to produce by the usualsteel-making and rolling processes, so the concentration x of Al afterdiffusion heat treatment in the range of the reduction rate of thelaminate was made x≧6.5 mass %. As explained above, by making thereduction rate α(%) of the laminate one in the above range in accordancewith the Al concentration, it is possible to produce a high Al-contentsteel sheet excellent in workability.

The range of the thickness t (mm) of the laminate after cold rolling inwhich particularly preferable effects can be obtained in the presentinvention may be illustrated as in FIG. 2. Here, the x (mass %) is theaverage concentration of Al after diffusion heat treatment. The reasonwhy the thickness t (mm) of the laminate after cold rolling is made0.015≦t≦0.4 is that if the thickness of the laminate after cold rollingis less than 0.015 mm, when cold rolling the laminate, the problemarises of partial delamination of the deposited Al or Al alloy and theyield falls, so the lower limit of t was made 0.015 mm. If over 0.4 mm,the Al diffusion heat treatment requires a high temperature or long heattreatment and the effect of the present invention of the improvement ofworkability after Al diffusion heat treatment becomes difficult toobtain, so the upper limit of t was made 0.4 mm. The reason for makingt≦−0.128x+1.298 was that the higher the Al concentration, the lower thetoughness of the steel sheet after diffusion heat treatment and thelower the workability, so to realize the effect of the present inventionof the excellent workability, the higher the Al concentration, thesmaller the laminate thickness must be made after cold rolling. Thepresent invention covers the method of production of steel sheetcontaining a high concentration of Al of 6.5 mass % or more, which isdifficult to produce by the usual steel-making and rolling processes, sothe concentration x of Al after diffusion heat treatment in the range ofthe thickness after cold rolling was made x≧6.5 mass %. As explainedabove, by making the thickness t of the laminate after cold rolling onein the above range in accordance with the desired average concentrationof Al after diffusion heat treatment, it is possible to produce a highAl-content steel sheet excellent in workability.

Here, cold rolling was mentioned as the optimal method of introducingworking strain, but even if the drawing method, shot peening method, oranother method able to impart working strain is used, the effect of thepresent invention can be obtained.

The temperature of the diffusion heat treatment is preferably 800° C. to1250° C. This is because if less than 800° C. or more than 1250° C., theeffect of the present invention of an excellent workability afterdiffusion heat treatment becomes difficult to obtain. A suitable time ofthe diffusion heat treatment is selected for causing the Al or Al alloydeposited on the surface to diffuse in the steel sheet. However, it isnot necessary to make the Al or Al alloy deposited on the surfaceuniform in the steel sheet. No matter what the heat treatmentatmosphere, that is, a vacuum atmosphere, Ar atmosphere, H₂ atmosphere,or other non-oxidizing atmosphere, the effect of the present inventionof improvement of the workability can be obtained.

Further to realize an even better effect of the present invention, thematrix steel sheet before deposition of Al or an Al alloy may bepreheated to 700° C. to 1100° C. This preheating is meant to rearrangethe dislocation structure formed in the process of production of thematrix steel sheet selected and preferably causes recrystallization, butthere is no particular need to cause recrystallization. If less than700° C., changes in the dislocation structure for obtaining the moreexcellent effect of the present invention become difficult. If over1100° C., an unpreferable oxide coating is formed on the surface of thesteel sheet. This has a detrimental effect on the subsequent depositionof Al or Al alloy and cold rolling, so the preheating was made 1100° C.was less. No matter what the atmosphere of this preheating, e.g., invacuum, an inert gas atmosphere, hydrogen atmosphere, or weakly acidicatmosphere, the above effects can be obtained, but conditions are soughtwhich would not form an oxide film on the surface of the steel sheetwhich would have a detrimental effect on the deposition of Al or an Alalloy after preheating or the subsequent cold rolling. The preheatingtime does not have to be particularly limited, but if considering theproducibility of high Al steel sheet, several seconds to several hoursare suitable.

In the above way, by preheating the matrix steel sheet and by depositingthe Al or Al alloy and cold rolling the laminate in the optimum ranges,the texture structure of the high Al-content steel sheet after thediffusion heat treatment falls within the range of the present inventionand the workability is remarkably improved compared with theconventional method of production. By using a high Al-content steelsheet produced in accordance with the present invention, it becomespossible to work a high Al-content steel sheet having a 6.5 mass % ormore Al concentration having a high temperature oxidation resistance andhigh electrical resistance, which was difficult to produce in the past,at a low cost. Therefore, the high Al-content steel sheet obtainedaccording to the present invention can be efficiently cold rolled by forexample a high reduction rate of 5% or more per pass and can be easilyworked into a metal foil of a thickness of 5 to 100 μm.

As explained above, the high Al-content steel sheet or high Al-contentmetal foil produced according to the present invention has excellentworkability and is useful for various applications such as automobilematerials, electrical heating materials, chemical plant materials, orpiping materials. Further, when using the metal foil using the method ofthe present invention for a metal substrate, it is useful for a metalsubstrate excellent in not only high temperature oxidation resistance,but also structural endurance and the delamination characteristic of thecatalyst layer.

The high Al-content steel sheet after diffusion heat treatment obtainedaccording to the present invention is not limited in chemicalcomposition other than the Al concentration, but a typical range ofcomposition (mass %) is Al: 6.5 to 10%, C: 2% or less, S: 0.1% or less,P: 0.1% or less, Si: 1% or less, Mn: 2% or less, and a balance of ironand unavoidable impurities and, in accordance with the characteristicssought from the final product, any alloy ingredient selected from Cr:30% or less, Ni: 15% or less, Mo: 2% or less, W: 2% or less, V: 2% orless, Ti: 5% or less, Nb: 2% or less, B: 0.1% or less, Cu: 1% or less,Co: 10% or less, Zr: 1% or less, Y: 1% or less, Hf: 1% or less, La: 1%or less, Ce: 1% or less, N: 0.1% or less, etc.

EXAMPLES

Below, examples will be used to explain the present invention in furtherdetail.

Example 1

In Example 1, the relationship of workability of a high Al steel sheetafter diffusion heat treatment with respect to the Al concentration ofthe matrix steel sheet was investigated. For the texture structure,X-ray diffraction was used to find the {222} plane integration parallelto the surface of the steel sheet and the {200} plane integration. Themethod of measurement was as follows. Table 1 shows the results.

The vacuum melting method was used to melt ingots of ingredientscontaining, by mass %, Al concentrations of 0, 1, 3, 3.5, 4.3, 5, 5.5,6, 6.4, and 7.0%, C, 0.007%, Si: 0.1%, Mn: 0.2%, Cr: 20%, Zr: 0.04%, La:0.1%, and a balance of iron and unavoidable impurities. Each was thenhot rolled to a 3.2 mm thickness, then cold rolled to a 0.4 mmthickness. The 0 to 6% Al materials could be produced without problem,but the 6.4% Al material partially suffered from hot rolling cracks. The7.0% Al material was low in toughness, so could not be produced. At thistime, the hot rolled sheet of the 7.0% Al material had a {222} planeintegration of less than 60% and a {200} plane integration of over15%—both not in the range of the present invention.

The matrix steel sheet having each Al concentration was plated with anAl alloy by the hot dipping method of Al. The plating bath had acomposition of 90% Al-10% Si. Al alloy was deposited on both surfaces ofthe steel sheet. Here, the deposited Al alloy was adjusted in thicknessas shown in Table 1 so that the Al concentration after diffusion heattreatment became constant. The laminate with the deposited Al alloy wascold rolled to a thickness of 0.1 mm. The reduction rate of the laminateof the sample of each Al concentration is shown in Table 1. After this,Al diffusion heat treatment was performed under conditions of 1000° C.and 2 hours under a reduced pressure hydrogen atmosphere to cause the Aldeposited on the surface to diffuse in the steel sheet. The average Alconcentration after the Al diffusion heat treatment was about 7.5 mass %for all samples. The workability was evaluated by running a tensile testat ordinary temperature and measuring the tensile elongation at break.The tensile test pieces were taken and tensile tests were conductedbased on JIS Z 2201 and JIS Z 2241 (in the following examples as well,the tensile tests were run based on the JIS standard).

As clear from the results of Table 1, it was learned that in InventionExamples 1 to 5 where the matrix steel sheets had Al concentrations of3.5 mass % to less than 6.5 mass %, the tensile elongations at breakwere improved. In Invention Examples 2 to 4 with Al concentrations of4.2 to 6.0%, particularly excellent elongations at break were exhibited.In each case, the {222} plane integration was 60% or more and the {200}plane integration was 15% or less or within the range of the highAl-content steel sheet of the present invention. On the other hand, inComparative Examples 1 to 3, the {222} plane integrations were less than60% and the {200} plane integrations were over 15% or not in the rangeof the present invention.

TABLE 1 Matrix Matrix steel sheet Al alloy deposition Evaluated Planeproduc- Al thickness Reduction rate α elongation at integration (%)Class ibility concentration/mass % per side/μm of laminate/% break/%{222} {200} Comp. Ex. 1 Good 0 53 80 2 45 23 Comp. Ex. 2 Good 1 45 79 251 21 Comp. Ex. 3 Good 3 30 78 3 53 18 Inv. Ex. 1 Good 3.5 26 78 10 62 8Inv. Ex. 2 Good 4.3 21 77 13 70 2 Inv. Ex. 3 Good 5 16 77 14 76 0.8 Inv.Ex. 4 Good 5.5 13 76 13 74 0.2 Inv. Ex. 5 Good 6 9 76 13 75 0.13 Inv.Ex. 6 Fair 6.4 7 76 10 73 0.8 Comp. Ex. 4 Poor 7 — — Not possible 37 26

Example 2

In Example 2, the effects of the reduction rate of the laminate withrespect to the average Al concentration after diffusion heat treatmenton the workability of the steel sheet after diffusion heat treatment wasinvestigated. Vacuum melting was used to melt ingots of ingredientsincluding, by mass %, C: 0.05%, Si: 0.3%, Mn: 0.2%, Al: 4.5%, Cr: 17%,and a balance of iron and unavoidable impurities. Each was hot rolled toa thickness of 3 mm, then cold rolled to a predetermined thickness. TheAl was deposited by hot dipping of Al. The plating bath had acomposition of 95% Al-5% Si. Al alloy was deposited on both surfaces ofthe steel sheet. After this, the laminate was cold rolled to thethickness described in Table 2. The thickness of the matrix steel sheetbefore deposition of the Al alloy and the thickness of the deposited Alalloy were adjusted to give the targeted average concentration of Alafter diffusion heat treatment and reduction rate of the laminate. Table2 describes the thickness of the matrix steel sheet before deposition ofthe Al alloy and the thickness of the deposited Al alloy per side. Foreach Al concentration, samples with constant thicknesses of thelaminates after cold rolling and different reduction rates wereprepared. After this, the samples were treated under the conditionsdescribed in Table 2 by Al diffusion heat treatment under an Aratmosphere to make the Al alloy deposited on the surfaces diffuse in thesteel sheets. The results of analysis of the average concentrations ofAl after the Al diffusion heat treatment are shown in Table 2. Theworkability was evaluated by conducting a tensile test at ordinarytemperature and measuring the tensile elongation at break.

X-ray diffraction was used to find the {222} plane integration parallelto the surface of the steel sheet and the {200} plane integration. Theresults are shown in Table 2. The method of measurement was as describedabove.

The sample with a reduction rate of 0% was produced by Al diffusion heattreatment without rolling the laminate, that is, by the conventionalmethod of production. In this case, the elongation at break was a low0.5% to 2%. The {222} plane integration was less than 60% and the {200}plane integration was over 15%—both of which not in the range of thepresent invention.

Next, at each of the Al concentrations, rolling the laminate improvedthe elongation at break compared with the conventional method of onlyperforming Al diffusion heat treatment.

When the average Al concentration after diffusion heat treatment was 6.5mass %, the elongation at break was greatly improved in the range of areduction rate of the laminate of 20% or more. When the Al concentrationwas 7 mass %, the elongation at break was greatly improved in the rangeof a reduction rate of the laminate of 22% or more. When the Alconcentration was 7.5 mass %, the elongation at break was greatlyimproved in the range of a reduction rate of the laminate of 22% ormore. When the Al concentration was 8.3 mass %, the elongation at breakwas greatly improved in the range of a reduction rate of the laminate of35% or more. When the Al concentration was 9.5 mass %, the elongation atbreak was greatly improved in the range of a reduction rate of thelaminate of 75% or more. Further, at each of the Al concentrations, witha reduction rate of the laminate of 96%, the problem arose of partialdelamination of the deposited Al.

From the above results, it is learned that if cold rolling a laminate onwhich an Al or Al alloy is deposited to impart working strain accordingto the present invention, the elongation at break after Al diffusionheat treatment is improved. Further, Table 2 shows the larger value ofthe lower limits of the reduction rate α(%), that is, α=35x−260 andα=20, with respect to the Al concentrations (mass %) for clarifying therange of the reduction rate where the effect of the present invention ismore remarkably realized. Cases where the reduction rate is in thisrange and 95% or less are shown with asterisks. From these results, whenthe reduction rate α is larger than the larger value of α=35x−260 andα=20, the elongation at break is 6% or more and a high Al-content steelsheet with excellent workability was obtained.

It could be confirmed that the high Al-content steel sheet produced bythe above method of the present invention was in the range of thepresent invention, that is, had one or both of a {222} plane integrationparallel to the steel sheet surface of 60% or more or a {200} planeintegration of 15% or less.

TABLE 2 Laminate Al Matrix thickness Average diffusion Al steel Al alloyt after Al heat diffusion Larger Plane sheet deposition cold Reductionconcen- treatment heat Evaluated of integration thick- thicknessrolling/ rate α of tration tempera- treatment elongation 35x − 260 (%)Class ness/mm per side/μm mm laminate/% x/mass % ture/° C. time/hr atbreak/% and 20 {222} {200} Comp. Ex. 5 0.33 10 0.35 0 6.5 1200 2 2 20 4520 Inv. Ex. 7 0.37 11 10 6.5 5 61 14 Inv. Ex. 8 0.41 12 *20 6.5 9 63 2.5Inv. Ex. 9 0.55 16 *40 6.5 12 72 0.7 Inv. Ex. 10 1.10 32 *70 6.5 13 750.4 Inv. Ex. 11 4.73 137 *93 6.5 12 85 0.16 Inv. Ex. 12 8.27 240 96 6.5♦12 88 0.1 Comp. Ex. 6 0.19 7 0.2 0 7 1100 2 1.5 20 42 23 Inv. Ex. 130.22 8 15 7 4 62 13 Inv. Ex. 14 0.24 9 *22 7 8 65 2 Inv. Ex. 15 0.37 14*50 7 10 73 0.64 Inv. Ex. 16 0.75 27 *75 7 12 75 0.35 Inv. Ex. 17 3.11113 *94 7 11 86 0.13 Inv. Ex. 18 4.66 170 96 7 ♦11 89 0.07 Comp. Ex. 70.18 8 0.2 0 7.5 1100 2 1.3 20 43 26 Inv. Ex. 19 0.22 9 15 7.5 3 63 11Inv. Ex. 20 0.24 10 *22 7.5 8 66 2.1 Inv. Ex. 21 0.37 16 *50 7.5 10 710.68 Inv. Ex. 22 0.92 40 *80 7.5 11 77 0.12 Inv. Ex. 23 2.63 115 *93 7.510 85 0.11 Inv. Ex. 24 4.60 202 96 7.5 ♦9 88 0.05 Comp. Ex. 8 0.09 5 0.10 8.3 1000 2 1 30.5 41 22 Inv. Ex. 25 0.13 7 30 8.3 3 62 12 Inv. Ex. 260.14 8 *35 8.3 7 68 3.5 Inv. Ex. 27 0.36 20 *75 8.3 9 73 0.56 Inv. Ex.28 1.12 63 *92 8.3 11 82 0.05 Inv. Ex. 29 2.25 125 96 8.3 ♦10 87 0.03Comp. Ex. 9 0.04 3 0.05 0 9.5 950 2 0.5 72.5 53 18 Inv. Ex. 30 0.07 5 409.5 2 61 12 Inv. Ex. 31 0.15 11 70 9.5 3 70 1.2 Inv. Ex. 32 0.17 13 *759.5 7 76 0.21 Inv. Ex. 33 0.73 54 *94 9.5 8 82 0.04 Inv. Ex. 34 1.09 8096 9.5 ♦7 88 0.03 1)*indicates invention examples included in range ofreduction rate of laminates where effects of the present invention areexpressed more remarkably. 2)♦indicates invention Examples 12, 18, 24,29, 34 have problems of partial delamination of Al during rolling oflaminate.

Example 3

In Example 3, the relationship of the workability of steel sheet afterdiffusion heat treatment in the case of holding the reduction rate ofthe laminate constant and different thicknesses of the laminate aftercold rolling with respect to the average Al concentration wasinvestigated.

The vacuum melting method was used to melt ingots of ingredientscontaining, by mass %, C, 0.01%, Si: 0.1%, Mn: 0.2%, Cr: 20%, Ti: 0.1%,Al: 5%, La: 0.03%, Ce: 0.03% and a balance of iron and unavoidableimpurities. Each was then hot rolled to a 3.2 mm thickness, then coldrolled to a predetermined thickness. The Al was deposited by hot dippingof Al. The plating bath had a composition of 90% Al-10% Si. Al alloy wasdeposited on both surfaces of the steel sheet. After this, the laminatewas cold rolled to the thickness described in Table 3. Here, thethickness of the steel sheet before deposition of the Al alloy and theamount of deposition of the Al alloy, like in Example 2, were adjustedto give the targeted average concentration of Al after diffusion heattreatment and reduction rate of the laminate. Table 3 describes thethickness of the steel sheet before deposition of the Al alloy and thethickness of the deposited Al alloy per side. After this, the sampleswere treated under the conditions described in Table 3 by Al diffusionheat treatment under an Ar atmosphere to make the Al alloy deposited onthe surfaces diffuse in the steel sheets. The results of analysis of theaverage concentrations of Al after the Al diffusion heat treatment areshown in Table 3. The workability was evaluated by conducting a tensiletest at ordinary temperature and measuring the tensile elongation atbreak.

X-ray diffraction was used to find the {222} plane integration and the{200} plane integration. The results are shown in Table 3. The method ofmeasurement was as described above.

When the Al concentration was 6.5 mass %, the elongation at break wasfurther greatly improved in the range of a laminate thickness after coldrolling of 0.4 mm or less. When the Al concentration was 7 mass %, theelongation at break was more improved in the range of a laminatethickness of 0.4 mm or less. When the Al concentration was 7.5 mass %,the elongation at break was more improved in the range of a laminatethickness of 0.3 mm or less. When the Al concentration was 8.5 mass %,the elongation at break was more improved in the range of a laminatethickness of 0.2 mm or less. When the Al concentration was 9.5 mass %,the elongation at break was improved at a laminate thickness of 0.05 mm.However, at all Al concentrations, when the laminate thickness aftercold rolling was 0.01 mm, the problem arose of partial delamination ofthe deposited Al.

Table 3 shows the smaller value of the upper limits of the laminatethickness t (mm), that is, t=−0.1283x+1.2983 and t=0.4, with respect tothe Al concentrations x (mass %) for clarifying the range of thethickness of the laminate after cold rolling where the effect of thepresent invention is more remarkably realized. Cases where the thicknessof the laminate after cold rolling is in this range and is 0.015 mm ormore are shown with asterisks. From these results, when the laminatethickness t (mm) after cold rolling is smaller than the smaller value oft=−0.1283x+1.2983 and t=0.4, the elongation at break becomes 9% or moreand a high Al-content steel sheet excellent in workability can beobtained. However, when the laminate thickness t (mm) is 0.01 mm, theproblem arose of partial delamination of the Al alloy when cold rollingthe laminate and the yield dropped.

It could be confirmed that the high Al-content steel sheet produced bythe above method of the present invention is in the range of the presentinvention having one or both of a {222} plane integration parallel tothe surface of the steel sheet of 60% or more or a {200} planeintegration of 15% or less.

TABLE 3 Laminate Al Matrix thickness Average diffusion Al steel Al alloyt after Al heat diffusion sheet deposition cold Reduction concen-treatment heat Evaluated Smaller Plane thick- thickness rolling/ rate αof tration tempera- treatment elongation of −0.1283x + integration (%)Class ness/mm per side/μm mm laminate/% x/mass % ture/° C. time/hr atbreak/% 1.2983 and 0.4 {222} {200} Inv. Ex. 35 0.72 16 0.45 40 6.5 12503 8.5 0.4 58 2.3 Inv. Ex. 36 0.64 14 *0.4 6.5 1200 13 72 0.2 Inv. Ex. 370.48 11 *0.3 6.5 1150 15 73 0.13 Inv. Ex. 38 0.32 7 *0.2 6.5 1100 15 760.09 Inv. Ex. 39 0.16 4 *0.1 6.5 1000 14 76 0.07 Inv. Ex. 40 0.08 2*0.05 6.5 950 14 77 0.08 Inv. Ex. 41 0.02 0.4 0.01 6.5 900 ♦13 79 0.06Inv. Ex. 42 0.85 26 0.45 50 7 1250 3 8 0.4 65 4.8 Inv. Ex. 43 0.75 23*0.4 7 1200 11 73 0.3 Inv. Ex. 44 0.38 11 *0.2 7 1100 12 76 0.2 Inv. Ex.45 0.19 6 *0.1 7 1000 13 78 0.11 Inv. Ex. 46 0.09 3 *0.05 7 950 12 800.1 Inv. Ex. 47 0.02 1 0.01 7 900 ♦12 81 0.09 Inv. Ex. 48 1.49 57 0.4 757.5 1200 3 7 0.336 67 5.3 Inv. Ex. 49 1.12 42 *0.3 7.5 1150 10 75 0.3Inv. Ex. 50 0.74 28 *0.2 7.5 1100 11 77 0.22 Inv. Ex. 51 0.37 14 *0.17.5 1000 12 79 0.17 Inv. Ex. 52 0.19 7 *0.05 7.5 900 13 81 0.1 Inv. Ex.53 0.04 1 0.01 7.5 900 ♦12 82 0.09 Inv. Ex. 54 1.13 61 0.25 80 8.5 11502 6.5 0.208 70 4.3 Inv. Ex. 55 0.90 48 *0.2 8.5 1100 9 78 0.8 Inv. Ex.56 0.45 24 *0.1 8.5 1000 10 80 6.6 Inv. Ex. 57 0.23 12 *0.05 8.5 950 1181 0.5 Inv. Ex. 58 0.05 2 0.01 8.5 950 ♦10 82 0.3 Inv. Ex. 59 0.44 300.1 80 9.5 1000 2 6 0.079 71 3.3 Inv. Ex. 60 0.22 15 *0.05 9.5 950 9 790.2 Inv. Ex. 61 0.04 3 0.01 9.5 950 ♦9 83 0.15 1)*indicates inventionexamples included in range of reduction rate of laminates where effectsof the present invention are expressed more remarkably. 2)♦indicatesinvention Example 41, 47, 53, 58, 61 have problems of partialdelamination of Al.

Example 4

In Example 4, the effect of preheating was investigated. The vacuummelting method was used to melt ingots of ingredients containing, bymass %, C: 0.01%, Cu: 0.4%, Al: 5.2%, Cr: 19%, Nb: 0.4%, and a balanceof iron and unavoidable impurities. Each was then hot rolled to a 3 mmthickness, then cold rolled to a thickness of 0.38 mm. After this, itwas preheated at the temperature shown in Table 4 for 30 minutes.However, Invention Example 62 was not preheated. Next, Al foil of athickness of 9 μm was bonded to both sides of the steel sheet to obtaina laminate. This laminate was then cold rolled to 0.1 mm. The reductionrate of the laminate at this time was 75%. After this, the laminate wastreated by Al diffusion heat treatment at 1000° C. for 2 hours in avacuum atmosphere to cause the Al foil bonded to the surfaces to diffusein the steel sheet. The result of analysis of the average concentrationof Al after the Al diffusion heat treatment was 7 mass % in each case.The workability was evaluated by conducting a tensile test at ordinarytemperature and measuring the tensile elongation at break. Further,X-ray diffraction was used to find the {222} plane integration parallelto the surface of the steel sheet and the {200} plane integration. Theresults are shown in Table 4. The method of measurement was as describedabove.

The not preheated sample and the sample preheated at 650° C. hadelongations at break of about 12%, while the samples preheated at 700°C. to 1100° C. had elongations at break greatly improved to 16 to 23%.Further, Invention Example 68 preheated at 1200° C. had the problem ofpartial delamination of the Al when cold rolling the laminate clad withAl foil. From the above results, it was confirmed that by preheatingaccording to the invention examples under a temperature of 700° C. to1100° C., the high Al steel sheet after Al diffusion heat treatment canbe further improved in workability. Further, it could be confirmed thateach of the high Al-content steel sheets is in the range of the highAl-content steel sheet of the present invention where a {222} planeintegration parallel to the surface of the steel sheet is 60% or more ora {200} plane integration is 15% or less.

TABLE 4 Evaluated Preheating elongation at Plane integration (%) Classtemperature/° C. break/% {222} {200} Inv. Ex. 62 — 12 72 0.8 Inv. Ex. 63650 13 73 0.7 Inv. Ex. 64 700 16 79 0.15 Inv. Ex. 65 800 23 83 0.08 Inv.Ex. 66 900 22 85 0.06 Inv. Ex. 67 1100 20 80 0.15 Inv. Ex. 68 1200 ♦1980 0.18 ♦indicates invention Example 68 had the problem of partialdelamination of Al

Example 5

In Example 5, the workability was evaluated under various conditions.

The method of production of the samples was similar to Example 3, butseveral samples were obtained by preheating a matrix steel sheet beforedeposition of an Al alloy at the preheating temperature shown in Table 5for 1 minute in a hydrogen atmosphere. Table 5 shows the productionconditions, that is, the thickness of the matrix steel sheet beforedeposition of the Al alloy, the preheating temperature, the thickness ofdeposited Al alloy per side, the thickness of the laminate after coldrolling, the reduction rate of the laminate, the Al diffusion heattreatment conditions, and the analysis value of the average Alconcentration of the steel sheet after Al diffusion heat treatment. Theworkability was evaluated in the same way as above by the tensileelongation at break. Further, X-ray diffraction was used to find the{222} plane integration parallel to the surface of the steel sheet andthe {200} plane integration. The results are shown in Table 5. Themethod of measurement was as described above.

When the Al concentration after Al diffusion heat treatment was 6.5 mass%, Comparative Example 10 with a reduction rate of the laminate of 0%had an elongation at break of 2%, while Invention Example 69 preheatedat 800° C. and with a reduction rate after deposition of Al of 50% and athickness of the laminate of 0.2 mm had an elongation at break of 22%.

When the Al concentration after Al diffusion heat treatment was 7.5 mass%, Comparative Example 11 with a reduction rate of the laminate of 0%had an elongation at break of 1.5%, while Invention Examples 70 to 72preheated at 750° C. and with reduction rates after deposition of Al of50, 75, and 88% and thicknesses of the laminate of 0.2, 0.1, and 0.05mm, that is, raised in reduction rate and reduced in thickness, hadelongations at break improved to 14, 16, and 18%.

When the Al concentration after Al diffusion heat treatment was 8.3 mass%, Comparative Example 12 with a reduction rate of the laminate of 0%had an elongation at break of 1%, while Invention Examples 73 to 76preheated at 850° C. and with reduction rates after deposition of Al of50, 75, 83, and 88% and thicknesses of the laminate of 0.2, 0.1, 0.07,and 0.05 mm, that is, raised in reduction rate and reduced in thickness,had elongations at break improved to 13, 14, 15, and 16%.

When the Al concentration after Al diffusion heat treatment was 9 mass%, Comparative Example 13 with a reduction rate of the laminate of 0%had an elongation at break of 0.5%, while Invention Examples 77 to 79preheated at 700° C. and with reduction rates after deposition of Al of75, 83, and 88% and thicknesses of the laminate of 0.1, 0.07, and 0.05mm, that is, raised in reduction rate and reduced in thickness, hadelongations at break improved to 12, 13, and 14%.

When the Al concentration after Al diffusion heat treatment was 9.5 mass%, Comparative Example 14 with a reduction rate of the laminate of 0%had an elongation at break of 0.5%, while Invention Example 80 preheatedat 800° C. and with a reduction rate after deposition of Al of 90% andthickness of the laminate of 0.02 mm had an elongation at break improvedto 11%.

In the case of the samples with the reduction rate of 0%, theelongations at break were low ones of 0.5% to 2%. In this case, the{222} plane integration was less than 60% and the {200} planeintegration was over 15%-neither within the range of the presentinvention. On the other hand, it could be confirmed that the highAl-content steel sheets produced by the method of the present inventionwere in the range of the present invention, that is, had one or both ofa {222} plane integration parallel to the steel sheet surface of 60% ormore or a {200} plane integration of 15% or less.

From the above results, it could be confirmed that by preheating thematrix steel sheet before deposition of an Al alloy and selecting thereduction rate of the laminate and the thickness after cold rolling inaccordance with the value of the average Al concentration x (mass %)after diffusion heat treatment to be in the range shown in FIG. 1 andFIG. 2, the high Al-content steel sheet after Al diffusion heattreatment was remarkably improved in elongation at break and exhibitedeven better workability of an elongation at break of 11% or more.

TABLE 5 Al Matrix Average diffusion Al steel Al alloy Thickness t Alheat diffusion sheet Preheating deposition of laminate Reduction concen-treatment heat Evaluated Plane thick- tempera- thickness after cold rateα of tration tempera- treatment elongation integration (%) Class ness/mmture/° C. per side/μm rolling/mm laminate/% x/mass % ture/° C. time/hrat break/% {222} {200} Comp. Ex. 10 0.19 — 4 0.2 0 6.5 1100 2 2 43 23Inv. Ex. 69 0.38 800 9 0.2 50 6.5 1100 2 22 83 0.08 Comp. Ex. 11 0.19 —7 0.2 0 7.5 1100 2 1.5 38 28 Inv. Ex. 70 0.37 750 14 0.2 50 7.5 1100 214 85 0.12 Inv. Ex. 71 0.37 750 14 0.1 75 7.5 1000 2 16 87 0.05 Inv. Ex.72 0.39 750 15 0.05 88 7.5 950 2 18 90 0.04 Comp. Ex. 12 0.18 — 9 0.2 08.3 1100 2 1 45 21 Inv. Ex. 73 0.36 850 18 0.2 50 8.3 1100 2 13 83 0.15Inv. Ex. 74 0.36 850 18 0.1 75 8.3 1000 2 14 85 0.1 Inv. Ex. 75 0.37 85019 0.07 83 8.3 950 2 15 88 0.05 Inv. Ex. 76 0.38 850 19 0.05 88 8.3 9002 16 93 0.03 Comp. Ex. 13 0.09 — 5 0.1 0 9 1000 2 0.5 52 18 Inv. Ex. 770.36 700 22 0.1 75 9 1000 2 12 86 0.11 Inv. Ex. 78 0.37 700 23 0.07 83 9950 2 13 89 0.06 Inv. Ex. 79 0.37 700 23 0.05 88 9 900 2 14 91 0.08Comp. Ex. 14 0.017 — 1 0.02 0 9.5 850 2 0.5 48 20 Inv. Ex. 80 0.176 80012 0.02 90 9.5 850 2 11 92 0.05

Example 6

In Example 6, the workability of the steel sheet after diffusion heattreatment was evaluated by whether cold rolling was possible and,further, the oxidation resistance was evaluated. In addition, the {222}plane integration of the high Al-content metal foil obtained after coldrolling and the {200} plane integration were measured and compared withthe evaluation of bendability of the metal foil performed later.

The samples of Comparative Examples 10, 11, 12, 13 and InventionExamples 69, 71, 74, 78 prepared in Example 5 were used to investigatewhether a high Al steel sheet after diffusion heat treatment could becold rolled to 0.02 mm. As shown in Table 6, the steel sheets ofComparative Examples 10, 11, 12, and 13 broke several times around theinitial period of rolling down to 0.02 mm and the rolling workabilitybecame remarkably poor.

The steel sheets of Invention Examples 69, 71, 74, 78 were all excellentin workability, so could be rolled down to 0.02 mm without breakage.

The {222} plane integration and the {200} plane integration of the highAl-content metal foil obtained by the cold rolling were not within therange of the present invention in any of Comparative Examples 10, 11,12, and 13. The {222} plane integrations or the {200} plane integrationsof the high Al-content metal foils of Invention Examples 69, 71, 74, and78 were within the range of the present invention. When bent bycorrugation, it was learned that the metal foils of the ComparativeExamples 10, 11, 12, and 13 suffered from numerous cracks at the bentparts and were inferior in workability. It was learned that with themetal foils of Invention Examples 69, 71, 74, and 78, no cracks wereseen and the workability was excellent.

The obtained 0.02 mm thick metal foil was cut into pieces, held in theatmosphere at 1100° C., and investigated for the oxidation resistance ofthe foil in terms of the time until causing abnormal oxidation where theincrease in mass per unit volume rapidly rises. Compared with theconventional foil (thickness of 10 μm) including Al in a concentrationof 5 mass % able to be produced by the usual steel-making and rollingprocesses, the time until abnormal oxidation started was extend to twotimes the 5% Al material in Invention Example 69 with 6.5% Al, 2.9 timesin Invention Example 71 with 7.5% Al, 3.7 times in Invention Example 74with 8.3% Al, and 4.4 times in Invention Example 78 with 9% Al.

As explained above, a high Al-content steel sheet obtained by control tothe range of the present invention can be easily worked into a thinmetal foil by cold rolling. Further, it is learned that high Al-contentmetal foils obtained by cold rolling further include a texture structurein the range of the present invention and have an excellent workability,so can be easily bent such as by corrugation. Further, these highAl-content metal foils have excellent resistance to high temperatureoxidation, so can be used for metal substrates etc.

TABLE 6 Average Al Thickness Thickness Plane integration of highconcentration before cold Cold after cold Al metal foil (%) Bendabilityof Class x/mass % rolling/mm reliability rolling/mm {222} {200} metalfoil Comp. Ex. 10 6.5 0.20 Poor 0.02 40 27 Poor Inv. Ex. 69 6.5 0.20Good 0.02 82 1.3 Good Comp. Ex. 11 7.5 0.20 Poor 0.02 37 29 Poor Inv.Ex. 71 7.5 0.10 Good 0.02 85 1.2 Good Comp. Ex. 12 8.3 0.20 Poor 0.02 4423 Poor Inv. Ex. 74 8.3 0.10 Good 0.02 83 1.8 Good Comp. Ex. 13 9.0 0.10Poor 0.02 51 19 Poor Inv. Ex. 78 9.0 0.07 Good 0.02 87 1.8 Good

Example 7

In Example 7, metal substrates were assembled and examined forstructural durability and adhesion of the catalyst layers. As the foilmaterials, a metal foil A of a thickness of 30 μm obtained by rolling asample of Invention Example 74 to 0.03 mm was prepared. Further, as acomparative example, a laminate metal foil B obtained by depositing a90% Si-10% Al alloy on both surfaces of a matrix steel sheet with an Alconcentration of 1.5 mass % and other additive elements etc. similar toInvention Example 74 and with a thickness of 0.36 mm by hot dipping to athickness of 42 μm a side, then rolling it to 0.03 mm without diffusionheat treatment was prepared. The foils A and B were corrugated andcoated with Ni-based brazing powder, then the corrugated foils and flatfoils were superposed and wound up to obtain 80 mm diameter honeycombstructures. These honeycomb structures were placed in outer jacketscomprised of ferrite-based stainless steel and heat treated them in avacuum at 1180° C. for 20 minutes. The foil A was brazed, while the foilB was brazed and simultaneously treated by diffusion heat treatment ofAl. The Al concentrations of the foil A and foil B were both 8.3 mass %.After that, catalyst layers of γ-Al₂O₃ powder impregnated with platinumcatalyst were formed, then the samples were subjected to a cooling andheating endurance test of repeated application of 1200° C. and 25° C.for 30 minutes each for 100 hours. The metal substrate using the foil Ashowed no abnormalities, while the metal substrate using the foil B hadmissing parts of foil, crushed honeycomb cells, and had chipped awayparts the surrounding catalyst layer. Due to the above, it is learnedthat the metal substrate comprised of the metal foil obtained by thepresent invention is excellent in toughness, so is also excellent instructural durability and the delamination characteristic of thecatalyst layer.

It is clear from the above that in the examples as well, the presentinvention gave high Al-content steel sheet excellent in workability.This high Al-content steel sheet can be easily worked into variousshapes and is high in toughness, so is useful for use as a metalsubstrate serving as the base material of for example an exhaust gaspurification catalyst substrate for an automobile.

INDUSTRIAL APPLICABILITY

By producing the high Al-content steel sheet of the present invention,the steel sheet after the diffusion heat treatment is remarkablyimproved in workability and working of a high Al-content steel sheethaving an Al concentration of 6.5 mass %, which had been difficult inthe past, can be performed at a low cost. The high Al-content steelsheet produced according to the present invention is excellent inworkability, so can be easily worked in various shapes and further canbe cold rolled to a thin foil. These are useful for the production ofvarious heat resistant materials etc. such as base materials forcatalyst substrates of automobiles etc. where for example hightemperature oxidation resistance is required.

The invention claimed is:
 1. A high Al-content steel foil having an Alcontent of 6.5 mass % to 10 mass % and a steel foil thickness of 0.005mm to 0.1 mm, said high Al-content steel foil characterized by having a{222} plane integration of 79% to 95% or a {200} plane integration of0.01% to 1.8% of the α-Fe crystal with respect to the surface of thesteel foil and an elongation at break of 11 to 23%, wherein said steelfoil is produced by a process comprising the steps of preheating amatrix steel sheet containing Al in an amount, by mass %, of 3.5% toless than 6.5% at a temperature of 750° C. to 1100° C., depositing Al orAl alloy on at least one surface of the matrix steel sheet to obtain alaminate, imparting working strain to the laminate by cold rolling,treating it by diffusion heat treatment, and further cold rolling thelaminate.
 2. A high Al-content steel foil as set forth in claim 1, saidhigh Al-content steel foil characterized by having both a {222} planeintegration of 79% to 95% and a {200} plane integration of 0.01% to 1.8%of the α-Fe crystal with respect to the surface of the steel foil.